Length measurement devices are typically utilized with machine tools for measuring the relative position of a tool with respect to a machined workpiece. In length measurement devices, which are utilized in machines tools, there occur measurement inaccuracies and/or errors which result from graduation errors, machining flaws, and by measuring errors which are caused by temperature variations.
Compensation of these inaccuracies and/or errors, which are caused by temperature variations, is required since, generally, in spaces containing machine tools, the ambient temperature is generally not constant. Also, the temperatures of most machine tools change constantly during the course of the machining. Because of cost considerations, the scale carrier with the scale, the carrier member for the scale carrier, and the object to be measured, which is usually in the shape of the associated machine part, are formed only in the rarest cases, from materials which have the same thermal expansion coefficients. Therefore, the graduation carrier, the carrier member, and the machine part are subjected to different thermal length changes at these unavoidable temperature changes of the ambient temperature. In this regard, measurement inaccuracies can occur during the course of the machining of a workpiece. This, however, given the present requirements relating to machining accuracy of workpieces, is no longer acceptable. Very often glass is used for the scale carrier, aluminum is used for the carrier member, and steel or grey cast iron is used for the machine part.
A length measurement device is known from the DE-OS 32 43 966, wherein the scale carrier, which has a scale which is connected to the carrier member by means of an elastic adhesive layer, has a fixed measuring reference point as its center while being longitudinally displaceable on both its sides in the measuring direction in case of a temperature change. The carrier member is, on one hand, connected at both ends by a respective length compensation element which has attachment members which are rigidly fastened to a machine part which is to be measured and, on the other hand, is rigidly fastened to the machine part at its center by an additional fastening element, so that the scale carrier with the scale retains the fixed measuring reference point relative to the machine part at its center during temperature changes.
An arrangement for a length measurement device for a machine tool is disclosed in DE-PS 31 06 701, wherein an elongation element, of a material having a higher thermal expansion coefficient than that of the machine part, is fastened with one of its ends to the machine part and with its other end to a scale carrier, which has a scale for compensating for the thermal length changes of a machine part. The thermal length change of the elongation bar is equal to that of the machine part so that the length change of the machine part is identified by the measuring device.
The two above-identified length measuring devices are however, tailored respectively only to a specific temperature compensation in a machine tool and, therefore, are unsuitable for temperature problems of different kinds at different machine tools.